/Engine/Source/Runtime/AdvancedWidgets/Public/Widgets/SRadialSlider.h
Set the SliderHandleStartAngle and SliderHandleEndAngle attributes. If the new SliderHandleStartAngle is more than the new SliderHandleEndAngle, SliderHandleEndAngle will be changed to equal SliderHandleStartAngle.
void SetSliderHandleStartAngleAndSliderHandleEndAngle ( float InSliderHandleStartAngle, float InSliderHandleEndAngle )